The present invention relates to flat image display devices employing feedback electron multiplication and specifically to a method for modulating the electron beam in such devices.
Several different types of devices have recently been suggested for producing a large area television display. One such device incorporates a plurality of electron multipliers each generating an electron beam which excites a different section of a cathodoluminescent screen. Coupled with the electron multipliers are modulating, focusing and accelerating electrodes to direct, shape and accelerate the electron beam toward the cathodoluminescent screen. The electron multipliers operate in a regenerative feedback mode. Specifically, the output of the multiplier, in addition to bombarding the screen, generates a feedback species, such as positive ions. The feedback species travel back to and strike the cathode of the multiplier thereby emitting secondary electrons and completing a feedback loop so as to produce sustained electron emission.
One of the difficulties in utilizing a plurality of individual electron sources to project an image is that such sources are intrinsically non-uniform in their output because of unavoidable variations in multiplier dimensions and secondary emission properties. As a result, the light output from the phosphor material of the cathodoluminescent screen, may vary from one area of the screen to another due to unequal excitation by the different electron beams. In addition, the flow of electrons from each multiplier must be modulated to provide the proper brightness level for the corresponding picture element of the display. If such devices are to be utilized for image display applications, such as television, a high degree of brightness uniformity and gray scale control are required.
To solve the brightness uniformity problem, one previous system sensed the charge build up on the screen due to the electron beam bombardment of each individual image element. The charge is directly related to the excitation of the phosphor elements and the brightness level of the emitted light. The electron beam in this system is cut off when a specified charge is reached which corresponds to the desired brightness level for that particular picture element. Although this system solved the uniformity problem, the rapid cut off of the electron beam required fast switching with relatively high circuit current levels. This requirement necessitated a relatively expensive switching circuit.